The invention relates to an improved method for providing a corrosive resistant coating to a magnesium-containing material. Specifically, the invention relates to an improved method for providing a chromium-silicon-containing, corrosive resistant coating to a magnesium-containing material.
Magnesium is a material that is commonly used in a variety of consumer and industrial materials such as automobile wheels and a variety of other automobile parts. However, magnesium is not entirely satisfactory for some purposes because it is subject to corrosion. A wide variety of corrosion resistant coatings have been developed for magnesium to make it more suitable for various uses.
Electrolytic coatings require a voltage to be applied between the substrate to be coated and a suitable counter electrode. The voltage acts as an external driving force to promote formation of the coating. Dip coatings do not require an external drawing force. In this case the substrate reacts directly with the coating solution.
It is known that protective layers of chromium oxides can be electrodeposited onto metal substrates to improve corrosion resistance. Such layers are known as chromium conversion coatings. At present, the production of chromium conversion coatings is carried out under acid conditions from a Cr.sup.+6 electrolyte containing sulfuric or nitric acids. These coatings contain Cr.sup.+6. These chromate coatings, however, are not entirely satisfactory because Cr.sup.+6 is highly toxic and the disposal of spent solutions is environmentally difficult.
British Pat. No. 1,531,056 describes electrolytic techniques for the production of chromium-containing conversion coatings based on Cr.sup.+3. Such coatings can be made containing substantially no Cr.sup.+6 and have been referred to as "chromite" coatings.
European Patent Application 0034040 teaches a Cr.sup.+3 conversion coating that can be applied by a nonelectrolytic method. A metal substrate is reacted with an aqueous solution comprising metal ions which are one or more of Cr.sup.+3, Fe.sup.+2, Fe.sup.+3, or Ni.sup.+2 ions in a concentration of up to 0.1 molar and an oxidizing agent to depolarize the reaction taking place at the surface of the substrate to be coated, whereby a coating is deposited on the surface of the substrate. Such coatings are referred to as "dip" coatings.
Dip coatings to deposit chromite conversion coatings onto the surface of metals are very useful and environmentally much safer than chromate coatings, however their deposition rate is relatively slow. A means for increasing the deposition rate would be highly desirable.
Likewise, the thickness to which chromite coatings can be applied is somewhat limited because the deposited coating blocks the deposition of additional thicknesses of coatings. It would be highly desirable to have a means to deposit thicker coatings of chromite onto metal substrates.
It is an object of the present invention to provide a method to rapidly deposit thick chromite dip coatings onto metal substrates.